In an image forming apparatus of this type, great fluctuations arise in electrical resistance values of a transfer unit, an image carrier, and a recording sheet to be nipped between the transfer unit and the image carrier depending on an ambient environment (particularly a temperature and a humidity). Accordingly, appropriate power supply must be provided according to changes in the ambient environment. For instance, when power supply becomes deficient, the force of adhesion of toner on the recording medium becomes insufficient, whereby the toner is left on the image carrier after transfer and the toner left after transfer is transferred to another position on the recording medium. Conversely, power supply becomes excessive, whereupon toner may splash or the image carrier may be damaged by electric discharge.
Japanese unexamined patent application publication No. JP-A-2006-53175 describes a related art image forming apparatus. The related image forming apparatus controls a transfer current to be applied to a transfer roller so as to come to an optimum value conforming to respective ambient environments. Specifically, a load resistance value of a transfer unit system including a transfer roller is measured, and an optimum value of a transfer current corresponding to the load resistance value is determined in accordance with a predetermined characteristic curve. The transfer current is controlled so as to come to this optimum value. The characteristic curve is a curve showing the correlation between respective load resistance values of the transfer unit system that change according to the ambient environment and optimum values of the transfer current corresponding to the respective load resistance values.